1. Field of the Invention
The present invention relates to a copolymer resin for a photoresist used with an ultra-shortwave light source such as KrF or ArF, a process for preparation thereof, and a photoresist using the same. More specifically, it relates to a photoresist resin wherein a 2,3-t-butyl-5-norbornene-2,3-dicarboxylate unit has been introduced into a norbornene-maleic anhydride copolymeric structure. The photoresist is preferably used in lithography processes using KrF(248 nm) or ArF(193 nm) light sources which are typically used in the manufacture of 1G or 4G Dynamic Random Access Memory (xe2x80x9cDRAMxe2x80x9d) semi-conductor integrated circuits.
2. Description of the Prior Art
In general, etching resistance, adhesiveness, and low light absorption at 193 nm wavelength are required for a copolymer resin to be useful as a photoresist for ArF light sources. The copolymer resin is also preferably developable by using 2.38 wt % aqueous tetramethylammonium hydroxide (TMAH) solution. However, it is very difficult to synthesize a copolymer resin satisfying all these properties. Up to the present time, many researchers have focused their studies on novolac type resins to increase transparency at 193 nm wavelength and increase etching resistance. Researchers at Bell Labs have tried to introduce an alicyclic unit to the backbone chain of the copolymer resin in order to enhance etching resistance. For example, a copolymer resin has been suggested in which the backbone chain has norbornene, acrylate and maleic anhydride substituents, as represented by the following Formula I: 
In the copolymer resin of Formula I, the maleic anhydride portion (identified by the letter A) is used for polymerizing the alicyclic olefin unit. However, the maleic anhydride portion is very soluble in 2.38% aqueous TMAH solution, even if it has not been exposed to the ArF light source, so that xe2x80x98top lossxe2x80x99 phenomenon (the top of the pattern being formed in a round shape) occurs in practical photoresist patterning. Thus, the copolymer resin of Formula I cannot be used as a photoresist for ArF sources.
In order to prevent such dissolution, it has been proposed to greatly increase the value of xe2x80x9cyxe2x80x9d in Formula I, thereby increasing the tert-butyl substituent. However, an increase in xe2x80x9cyxe2x80x9d causes a relative decrease in the value of xe2x80x9czxe2x80x9d. The relative decrease of the xe2x80x9czxe2x80x9d portion, which enhances sensitivity and adhesiveness with the substrate, causes disadvantages in practical patterning in that the photoresist tends to be removed from the wafer so that a pattern cannot be formed.
In accordance with one aspect of the present invention, a 2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate unit having two protective groups is introduced into a norbornene-maleic anhydride copolymer structure, resulting in a copolymer resin having a high transparency at 193 nm wavelength and high etching resistance. The copolymers of the present invention can be easily prepared by conventional radical polymerization, and prevent top loss phenomenon, exhibit enhanced adhesive strength and show excellent resolution of 0.13 xcexcm in patterning applications.
It is an object of the present invention to provide a copolymer resin comprising a 2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate unit.
It is another object of the present invention to provide a process for preparing the copolymer resin comprising a 2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate unit.
It is still another object of the present invention to provide a photoresist comprising the above norbornene-maleic anhydride copolymer resin.
It is still further object of the present invention to provide a semiconductor element manufactured by using the above copolymer resin as a photoresist.
The present invention relates to a copolymer resins comprising a 2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate unit, represented by the following Formula III: 
The copolymer resins according to the present invention preferably include the following norbornene-maleic anhydride copolymer resins:
(1) Poly[2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate/2-hydroxyethyl-5-norbornene-2-carboxylate/maleic anhydride/5-norbornene-2-carboxylic acid] copolymer resin (molecular weight: 3,000-100,000) having the following Formula IV: 
wherein, the ratio x:y:z is (0.001-99%):(0-99%):(0-99%);
(2) Poly[2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate/2-hydroxyethyl-5-norbornene-2-carboxylate/maleic anhydride/mono-methyl cis 5-norbornene-endo-2,3-dicarboxylic acid] copolymer resin (molecular weight: 3,000-100,000) having the following Formula V; 
wherein, the ratio x:y:z is (0.001-99%): (0-99%): (0-99%);
(3) Poly[2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate/2-hydroxypropyl-5-norbornene-2-carboxylate/maleic anhydride/5-norbornene-2-dicarboxylic acid] copolymer resin (molecular weight: 4,000-100,000) having the following Formula VI: 
wherein, x is a mole % of 0.001-99%, more preferably is 0.5-99%, and y and z are independently a mole % of 0.1-99%; and
(4) Poly[2,3-di-t-butyl-5-norbornene-2,3-dicarboxylate/2-hydroxypropyl-5-norbornene-2-carboxylate/maleic anhydride/mono-methyl cis 5-norbornene-endo-2,3-dicarboxylate] copolymer resin (molecular weight: 4,000-100,000) having the following 
wherein, the ratio x:y:z is (0.001-99%): (0-99%): (0-99%).
The copolymer resin of Formula IV can be prepared according to the present invention by reacting 2,2-di-t-butyl-5-norbornene-2,3-dicarboxylate, 2-hydroxyethyl-5-norbornene-2-carboxylate, maleic anhydride and 5-norbornene-2-carboxylic acid in the presence of a conventional polymerization initiator, as below illustrated in Reaction Scheme I. 
The copolymer resin of Formula V can be prepared according to the present invention by reacting 2,2-di-t-butyl-5-norbornene-2,3-dicarboxylate, 2-hydroxyethyl-5-norbornene-endo-2,3-dicarboxylic acid, maleic anhydride and mono-methyl cis-5-norbornene-endo-2,3-dicarboxylic acid in the presence of a conventional polymerization initiator, as below illustrated in Reaction Scheme II. 
The copolymer resin of Formula VI can be prepared according to the present invention by reacting 2,2-di-t-butyl-5-norbornene-2,3-dicarboxylate, 2-hydroxypropyl-5-norbornene-2-carboxylate, maleic anhydride and 5-norbornene-2-carboxylic acid in the presence of a conventional polymerization initiator, as below illustrated in Reaction Scheme III. 
The copolymer resin of Formula VII can be prepared according to the present invention by reacting 2,2-di-t-butyl-5-norbornene-2,3-dicarboxylate, 2-hydroxypropyl-5-norbornene-2-carboxylate, maleic anhydride and mono-methyl cis-5-norbornene-endo-2,3-dicarboxylic acid in the presence of a conventional polymerization initiator, as below illustrated in Reaction Scheme IV. 
The aforementioned copolymer resins (Formulas IV to VII) can be prepared according to the present invention by a conventional polymerization process such as bulk polymerization or solution polymerization. Preferred polymerization initiators used in the present invention include benzoyl peroxide, 2,2xe2x80x2-azobisisobutyronitrile(AIBN), acetylperoxide, laurylperoxide, tert-butylperacetate, di-t-butyl peroxide, or the like. As a solvent, cyclohexanone, methyl ethyl ketone, benzene, toluene, dioxane, and/or dimethylformamide may be used individually, or in a mixture.
In the process for preparing the copolymer resin according to the present invention, general polymerization conditions, including temperature and pressure of radical polymerization, may vary depending upon the properties of the reactants, but it is preferable to carry out the polymerization reaction at a temperature between 60xc2x0 C. and 200xc2x0 C. under nitrogen or argon atmosphere for 4 to 24 hours.
The copolymer resin photoresist composition according to the present invention can be prepared using a conventional process for forming a photoresist composition; that is, by mixing the resin with a conventional photoacid generator in the presence of an organic solvent to form a photoresist solution. This photoresist composition can be used in the formation of a positive micro-image. In the process of forming a photoresist pattern on a semiconductor element, the amount of the copolymer resin depends on the organic solvent or photoacid generator used, and the conditions of lithography, but conventionally it is about 10 to 30% by weight of the organic solvent used.
The process for forming a photoresist pattern of a semiconductor element by using the copolymer resin according to the present invention is described in detail hereinbelow:
The copolymer resin of the present invention is preferably dissolved in cyclohexanone at a concentration of 10 to 30% by weight. An onium salt or organic sulfonic acid (0.1 to 10% by weight of copolymer resin) is preferably added to the copolymer resin solution as an photoacid generator. The mixture is filtered through an ultra-micro filter to prepare the photoresist solution. Preferred photoacid generators include triphenylsulfoniumtriflate, dibutylnaphtylsulfoniumtriflate, 2,6-dimethylsulfonate, bis(arylsulfonyl)-diazomthane, oximsulfonate, 2,1-diazonaphthoquinone-4-sulfonate, and the like.
Preferred organic solvents other than cyclohexanone, include ethyl 3-ethoxypropionate, methyl 3-methoxypropionate, propyleneglycol methyl ether acetate, and the like. It is preferable to use the solvent in a quantity equal to 200 to 1000 wt % of the photoresist resin used in the preparation.
The photoresist solution is preferably spin-coated on a silicon wafer to form a thin film, and the coated wafer is then prebaked in an oven or on a hot plate at 80xc2x0 C.-150xc2x0 C. for 1-5 minutes, exposed to light by using a far ultraviolet exposer or an eximer laser exposer, and post-heated at 100xc2x0 C.-200xc2x0 C. for 1 second to 5 minutes. The exposed wafer is impregnated with 2.38% aqueous TMAH solution for 1-1.5 minutes to obtain a positive resist pattern.
The light source used to expose the photoresist can be ArF, KrF, E-beam, EUV (extreme ultraviolet), ion beam, or the like. Preferred light energy for exposure is 0.1-10 mJ/cm2.
A better understanding of the present invention may be obtained by reference to the following examples which are set forth to illustrate, but are not to be construed to limit, the present invention.